The present invention relates generally to the field of radio communication, and more specifically, to the field of output power control in code division multiple access (CDMA) cellular telephones.
Several industry standard publications currently direct design and operation of all types of CDMA cellular telephones, including portable mobile stations, handheld mobile stations, and mobile stations mounted in automobiles. These standards are considered to be understood by those reasonably skilled in the art of the present invention. Standard specifications relevant to the present invention include TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, sections 6.1.1.1-6.1.2.4.2, and TIA/EIA/IS-98, Recommended Minimum Performance Standards for Dual-Mode Wideband Spread Spectrum Cellular Mobile Stations, sections 1.4, 10.4.4.1-10.5.2.3.
Precise mobile station power control is a very important requirement for proper and efficient operation of a CDMA cellular telephone system. During times when a mobile station is located far away from the nearest base station, the mobile station needs to transmit signals at maximum output power to maintain an optimum communication link. However, as a CDMA mobile station moves closer to a base station, the amount of mobile station output power necessary to continue communication is reduced. Furthermore, such a reduction in mobile station output power is necessary to reduce interference between mobile stations. Thus, in addition to preserving battery reserves, the use of as little mobile station output power as is necessary to maintain a strong communication link at all times is a requirement to ensure proper operation of a CDMA cellular telephone system.
According to the above-referenced standards, a CDMA mobile station shall provide two independent means for output power adjustment: open loop estimation, solely a mobile station operation, and closed loop correction, involving both the mobile station and the base station. An open loop gain control system detects the strength of the signal received from the base station and uses that value to inversely control the output power of the mobile station. Thus, according to the open loop gain control system, as the received signal strength increases (the mobile station nearing the base station), the amount of output power is decreased. A typical open loop gain control system includes a conventional automatic gain control system which detects received signal strength and uses that value to control an adjustable gain transmitter amplifier which accordingly varies the amount of gain applied to the radio transmission signal. Thus, the open loop gain control system is solely a mobile station operation based upon the strength of the signal received at the mobile station from the base station.
A closed loop gain control system, on the other hand, involves both the mobile station and the base station. In a closed loop gain control system, the base station detects the strength of the signal received from the mobile station and then accordingly instructs the mobile station to increase or decrease power. Thus, the mobile station continually analyzes control data from the base station (typically in the form of a single bit commanding either an increase or a decrease in power) to determine whether to increase or decrease output power. A closed loop gain control system is typically implemented using a closed loop power control register which is functionally connected to a pulse density modulator within a mobile station modem application-specific integrated circuit (MSM ASIC) to provide an analog output representation of the value stored in the register. This analog representation is then combined with output from the open loop gain control system to assist in controlling the adjustable transmitter amplifier. The register value and adjustable transmitter amplifier are also usually in an inverse relationship such that an increase in the register value (typically due to receiving a "1" from the base station) results in a decrease in overall power.
Achieving proper operation of an output power control system incorporating both open and closed loop gain control systems within a single mobile station can create special design challenges. In addition to the expected difficulties encountered when combining two such independent systems, other problems are created by additional requirements imposed by the above-referenced standards. Namely, there are limits on total power output and spurious emission levels, requirements for closed loop variations about the open loop estimate, and response time requirements for responding to instructions from the base station. First, using a handheld mobile station operating at full rate communication, the effective radiated power at maximum output power has an upper limit of 30 dBm and a lower limit of 23 dBm. Thus, when at maximum power, the mobile station must radiate at least 23 dBm but not more than 30 dBm. The actual value for effective radiated power at maximum output power is typically around 28 dBm because of the second requirement which limits maximum spurious emission levels. On the lower end of the total output power scale, the mobile station must have a mean controlled output power less than -50 dBm when the output power is set to minimum. Thus, according to industry standard specifications, the mobile station should ideally be able to output power throughout a range of -50 dBm to 30 dBm.
In addition to these requirements, the closed loop gain control system must have a range of at least 24 dB above and 24 dB below the open loop estimate. In other words, regardless of where the open loop estimate places the total output power along the -50 dBm to 30 dBm range, the closed loop gain control system is required to be able to increase or decrease the total output power by at least 24 dB upward or downward from that open loop estimate. In one implementation of the closed loop gain control system, the closed loop range is divided into equal steps represented by incremental memory values corresponding to one dB units of gain. Thus, as the base station instructs the mobile station to increase or decrease power, the closed loop gain control system attempts to increase or decrease, respectively, the total output power by one dB.
Clearly, these requirements related to total output power and closed loop gain control create the potential for conflict. In other words, if the open loop estimate is within 24 dB of the maximum output power (e.g., above 6 dBm in an ideal 30 dBm system), the potential exists for the closed loop system to enter into a saturated condition. For instance, if the mobile station is far from the base station, the base station may continue to instruct the mobile station to increase power even after the mobile station has reached its maximum output power. In such a situation, the closed loop system could be at least 24 steps above the maximum output ability of the mobile station so that it would take 24 consecutive decrease power instructions from the base station before the mobile station would begin to reduce actual output power. Unfortunately, this result is not acceptable in light of yet another requirement imposed by the above-referenced standard which dictates closed loop responsiveness. According to the responsiveness requirement, after a mobile station receives a reduce power instruction from the base station, the mobile station must begin reducing power within a short defined amount of time. When communicating at full rate, this amount of time is 2.5 ms. Since, at full rate, power control instructions arrive from the base station every 1.25 ms, total output power is required to begin decreasing by the time two subsequent valid power control instructions are received from the base station. Since, as discussed above, it may take 24 or more steps to pull the closed loop gain control system out of saturation, conventional power control systems may be unable to satisfy the closed loop responsiveness requirement.
Developing an efficient and reliable solution to this problem which works in all situations yet doesn't prevent the resulting system from satisfying the other requirements is not an obvious process. Part of the difficulty rests with the fact that the open loop system also often continues to increase after the maximum total output power has been reached. Furthermore, large temporary obstacles, such as buildings, require rapid modifications to the output power. Thus a system is needed which is able to meet the industry standard specifications in rapidly changing conditions. Finally, there is also the need that the closed loop range be greater than plus or minus 24 dB to compensate for component tolerances in production and other potential variations. Unfortunately, addressing this need by expanding the range creates the potential for even greater saturation of the closed loop gain control system.
There is, therefore, a need in the industry for a method and an apparatus for addressing these and other related, and unrelated, problems.